What you were expecting to happen, and what actually happened
In PvP Scylla casted and submerged all allies, when being hit by an opponents cast, allies move and turn back into their position. In this case the ally moved onto the board and did not return.

How often does this happen? When did it begin happening?
This is happening sometimes, not reproducable. Would say 1 out of 20 matches.

Steps to make it happen again
No chance to reproduce, only seen in PvP so far.

Happened to enemy’s Coral Golem on Bounty Hunt as well. He stayed in the middle of the board, matching hidden skulls under him to kill my troops. Sneaky bastard… My Rift Lynx managed to knock him back, to stuck in new place at between slot 3 and 4. = =

My guess how it happen is when top submerged troop matching skull to attack, but somehow stucked on its return trip. If I recall correctly, it’d been happening since submerge is introduced. The dev know about it, but hard to reproduce, so still don’t know its main cause, I think.